The present invention relates to the improvement in an electric discharge machining power supply system employed in the electric discharge machining, which executes the machining of the workpiece by generating the electric discharge between the electrode and the workpiece as the electrode, to supply a machining power between the electrodes.
The electric discharge machining generates the electric discharge between electrodes by supplying the DC voltage between the electrode and the workpiece from the DC power supply of the electric discharge machining power supply system to proceed the machining while moving the workpiece relatively to the electrode.
The DC voltage supplied between the electrode and the workpiece is controlled by turning ON/OFF the switching means. The inter-electrode voltage does not quickly fall down because of the presence of the stray electrostatic capacitance between the electrodes and the inductance of the circuit, etc. after the switching means is turned OFF, and thus the situation that the voltage still remains between the electrodes is caused. In case the electric discharge is not generated at the end of the voltage pulse, the remaining voltage between the electrodes immediately after the turning OFF of the switching means is substantially equal to the applied voltage and the voltage that is in excess of the electric discharge starting voltage still remains between the electrodes. Thus, there is the possibility that the electric discharge is generated.
Also, an amount of energy stored by the stray electrostatic capacitance between the electrodes and the inductance of the circuit, etc. is changed depending on the size of the workpiece, the machining condition, the gap between the electrodes, the length of the lead wire of the circuit, etc. and is not constant. Thus, since a time required to consume the energy is changed and the energy at the generation of the electric discharge becomes different, it is difficult to realize the stable machining.
The electric discharge generated due to the remaining voltage between the electrodes in the quiescent time from the end of the voltage application between the electrode (OFF of the switching means) to the subsequent voltage application (ON of the switching means) is different from the electric discharge in the time period in which the proper electric discharge is intended, and therefore such electric discharge is out of the control range. This remaining voltage between the electrodes is changed gradually with the falling of the voltage and thus an amount of supplied energy is not fixed. Thus, there is the problem that the machining quality is lowered.
In addition, there is the problem that, when the relative movement between the electrode and the workpiece is executed by the inter-electrode voltage, the electric discharge machining cannot be carried out at a desired gap between the electrodes because of the presence of the remaining voltage between the electrodes.
Further, the electrostatic force is generated between the electrode and the workpiece by the remaining voltage between the electrodes. In particular, in the case of the wire electric discharge machining, there is the problem that the machining precision of the workpiece is lowered by the vibration of the wire electrode due to this electrostatic force.
FIG. 7 is a block diagram showing a configuration of the electric discharge machining power supply system in the prior art. In FIG. 7, 1 is an electrode, 2 is a workpiece, 3 is a DC power supply for supplying the DC voltage between the electrode 1 and the workpiece 2, 4 is a switching means, 5 is a controlling means for controlling ON/OFF of the switching means 4, and 6 is a gap resistance. A structure in which the remaining energy between the electrodes is consumed by adding the gap resistance 6 between the electrode 1 and the workpiece 2 is employed.
FIG. 8 is a view showing an inter-electrode voltage waveform in the electric discharge machining power supply system having the circuit configuration shown in FIG. 7 in the prior art. In FIG. 8, V is an inter-electrode voltage, T1 is a voltage pulse applying time, and Tr is a quiescent time. Since the inter-electrode voltage V is reduced like an exponential function after the switching means 4 is turned OFF, it is impossible to render the inter-electrode voltage V to fall down quickly.
In this manner, according to the method of consuming the energy remaining between the electrodes by the gap resistance between the electrodes, since a time is required to consume the remaining energy between the electrodes, it is difficult to shorten the quiescent time. In addition, there still remains the possibility that the electric discharge is generated on the outside of the control range until the voltage is reduced below the electric discharge starting voltage.
Besides, in the Unexamined Japanese Patent Application Publication No.Heil-257513, the electric discharge machining power supply system which is capable of improving the machining quality of the workpiece and also preventing the electrolytic corrosion by changing the polarity of the voltage pulse every predetermined number of the voltage pulses is disclosed. But there is no disclosure to cause the voltage pulse between the electrodes to fall down quickly. Since the voltage remaining between the electrodes does not quickly fall down after the end of voltage application, there is the time in which the voltage is in excess of the electric discharge starting voltage. There are the problems that it is difficult to suppress the generation of the electric discharge during this time, etc.
The present invention has been made to overcome above problems, and it is an object of the present invention to obtain an electric discharge machining power supply system capable of achieving the stable electric discharge machining having high machining quality.
Also, it is another object of the present invention to obtain an electric discharge machining power supply system capable of preventing the electrolytic corrosion of the workpiece and capable of shortening the machining time.
An electric discharge machining power supply system according to the present invention comprises a DC power supply for supplying a DC voltage between an electrode and a workpiece, switching means for switching the DC voltage, and a controlling means for controlling ON/OFF of the switching means, whereby a voltage of both positive and negative polarities can be applied between electrodes, wherein the controlling means applies a first voltage pulse between the electrodes for a predetermined time, and then applies a second voltage pulse having a polarity opposite to the first voltage pulse for a predetermined time until the voltage between the electrodes is lowered to a predetermined voltage that is less than an electric discharge starting voltage.
Also, an electric discharge machining power supply system according to the present invention comprises a DC power supply for supplying a DC voltage between an electrode and a workpiece, switching means for switching the DC voltage, and a controlling means for controlling ON/OFF of the switching means, whereby a voltage of both positive and negative polarities can be applied between electrodes, wherein the controlling means applies a first voltage pulse between the electrodes for a predetermined time, and then applies a second voltage pulse having a polarity opposite to the first voltage pulse at a predetermined period for a predetermined time such that the voltage between the electrodes is within a predetermined voltage range that is less than an electric discharge starting voltage.
Also, an electric discharge machining power supply system according to the present invention comprises a DC power supply for supplying a DC voltage between an electrode and a workpiece, switching means for switching the DC voltage, and a controlling means for controlling ON/OFF of the switching means, whereby a voltage of both positive and negative polarities can be applied between electrodes, the system further comprises a voltage detecting means for detecting the voltage between the electrodes, a voltage comparing means for comparing an inter-electrode voltage detected by the voltage detecting means with a predetermined voltage that is set lower than an electric discharge starting voltage, and a controlling means for controlling ON/OFF of the switching means, that apply a second voltage pulse having a polarity opposite to the first voltage pulse such that the inter-electrode voltage is set within a predetermined voltage range that is less than an electric discharge starting voltage, based on a compared value obtained by the voltage comparing means after a first voltage pulse is applied between the electrodes for a predetermined time.
Also, in the electric discharge machining power supply system according to the present invention, after application of the first voltage pulse is ended, the second voltage pulse is applied after a predetermined time has lapsed to prevent breakdown of the switching means.
Also, an electric discharge machining power supply system according to the present invention comprises a DC power supply for supplying a DC voltage between an electrode and a workpiece, switching means for switching the DC voltage, and a controlling means for controlling ON/OFF of the switching means, whereby a voltage of both positive and negative polarities can be applied between electrodes, the system further comprises a voltage detecting means for detecting the voltage between the electrodes, a voltage comparing means for comparing an inter-electrode voltage detected by the voltage detecting means with a predetermined voltage that is set lower than an electric discharge starting voltage, an average voltage detecting means for detecting an average voltage between the electrodes, an average voltage comparing means for comparing the average voltage between the electrodes detected by the average voltage detecting means with a zero bolt, and a controlling means for controlling ON/OFF of the switching means, that apply a second voltage pulse having a polarity opposite to the first voltage pulse such that the inter-electrode voltage is set within a predetermined voltage range that is less than an electric discharge starting voltage, based on a compared value obtained by the voltage comparing means after a first voltage pulse is applied between the electrodes for a predetermined time, controlling ON/OFF of the switching means, that apply a fourth voltage pulse having a same polarity as the first voltage pulse such that the inter-electrode voltage is set within a predetermined voltage range that is less than the electric discharge starting voltage, based on the compared value obtained by the voltage comparing means after a third voltage pulse having a polarity opposite to the first voltage pulse is applied between the electrodes for a predetermined time, and controlling the average voltage between the electrodes at a predetermined time interval into zero, based on the compared value obtained by the average voltage detecting means.
Also, in the electric discharge machining power supply system according to the present invention, the predetermined voltage that is less than the electric discharge starting voltage when the second voltage pulse is applied or the predetermined voltage that is less than the electric discharge starting voltage when the fourth voltage pulse is applied is set close to the electric discharge starting voltage.
Also, in the electric discharge machining power supply system according to the present invention, after application of the first voltage pulse is ended, the second voltage pulse is applied after a predetermined time has lapsed to prevent breakdown of the switching means, and, after application of the third voltage pulse is ended, the fourth voltage pulse is applied after a predetermined time has lapsed to prevent breakdown of the switching means.
Since the electric discharge machining power supply system according to the present invention is constructed as above, effects described in the following can be achieved.
In the electric discharge machining using the electric discharge machining power supply system according to the present invention, there can be achieved the effect that the stable machining having high machining quality can be executed effectively. In particular, if the wire electric discharge machining is employed in the electric discharge machining power supply system according to the present invention, the application frequency of the voltage pulse can be increased because the vibration of the wire electrode due to the electrostatic force can be suppressed, the electric discharge due to the remaining voltage between the electrodes can be suppressed, the feed of the wire electrode can be stabilized, etc. Therefore, there can be achieved the effect that the more effective wire electric discharge machining having high machining quality can be implemented.
In the electric discharge machining using the electric discharge machining power supply system according to the present invention, there can be achieved the effect that not only the electrolytic corrosion of the workpiece can be prevented but also the machining time can be shortened.